Manufacturers of cast-metal components having machined surfaces typically inspect each component to identify presence of surface defects, to determine if the component is in compliance with quality standards. Surface defects of interest typically include pores, or openings, which indicate voids in the cast metal. Such voids are caused, for example, by impurities in the cast metal, and metal flow problems during the casting process. A specific pore may indicate a small void, or it may indicate and lead to a much larger void below the surface. A pore may be located in a critical location in the cast part, such as an edge of a casting, or near a heat-affected zone, and affect overall part quality and performance in-use. Alternatively, a pore may be located in a non-critical location, with little or no effect on part quality and in-use reliability and performance. Voids in the casting typically associated with surface defects include, by way of example, a small isolated opening, a pore with a concave region, a convex hull in the casting created prior to pore separation, a top edge-connected pore, or, an edge-connected pore on the inside of a concave region.
One present method and apparatus for inspecting and detecting surface defects comprises having a human operator measure a machined surface using a MYLAR™ (or equivalent) template, and visually evaluating the result to identify and detect defects therefrom. The quality and reliability of such inspection is subject to the operator's overall capability and performance over time.
A second method and system as been developed comprising a surface porosity inspection process using a line-scan camera and lighting system to obtain an image of a surface. Software was developed to evaluate surface pores using adaptive threshold and characterization of isolated and edge-connected pores. An adaptive threshold grayscale process generates a black and white image of the surface. An inverted binary (i.e. pure black and white) image is presented for processing to identify the pores using algorithms. Such a system is not robust in a manufacturing production environment because of limitations related to identification of surface defects with edge-connected pores. These limitations included an inability to reliably detect a pore with a concave region, a convex hull created prior to pore separation, or an edge-connected pore on an inside of a concave region. In addition, size of convexity defects and neighboring pores may be inaccurately measured, leading to false detections. Furthermore, the system may falsely detect small defects, or sharp features, or tightly-cornered features. Such a system typically requires an operator to inspect and sort parts after machine inspection in conjunction with the analysis from the inspection system, defeating the purpose of the inspection station, and introducing risk of human error into the process.
Therefore, there is a need to objectively inspect and analyze a surface of a machined, cast-metal part at manufacturing line processing rates to accurately, repeatably, and reliably identify defects so as to minimize and eliminate false-pass errors. There is a further need to reliably identify intricately-shaped defects and high-density small-sized defects. There is a further need to minimize operator involvement in the inspection process, and improve part throughput rate of the inspection process.